Coatings and material layers are often applied to optical elements such as ophthalmic lenses, sunglasses, visors, windshields, etc. for controlling the level of light passing through these elements. There are two common types of sunglasses that use such special coatings and materials.
The first type are the “anti-glare” sunglasses. On sunny days, objects in one's field of vision often reflect sunlight specularly. This reflection is far brighter than the diffusely reflected light from the same object, rendering the object difficult to see. These reflections are commonly called “glare.” Presently, anti-glare coatings to reduce these undesirable seeing conditions are dichroic. In other words, they preferentially absorb light having a particular plane of linear polarization. The original Polaroid H sheet (U.S. Pat. No. 2,319,816) was a dichroic polarizer. It functions by absorbing (and hence, removing) light with direction of polarization perpendicular to its polarizing axis, and allowing to pass light that has direction of polarization parallel to its polarizing axis. Such coatings perform their anti-glare role because specularly-reflected light is partially linearly polarized and, thus, can be absorbed by a dichroic layer, properly oriented, on the optical element before the wearer's eyes. The diffusely-reflected light is randomly polarized, and so is only very weakly altered by the dichroic coating.
The second type are the “photochromic” sunglasses. Their apparent color (the amount of light they absorb at a particular wavelength or wavelengths) reversibly changes in response to the intensity of light with which they are illuminated. Typically, the photochromic reaction is in response to bright ultraviolet illumination, while the enhanced absorption is at visible wavelengths. These devices rely on a reversible photo induced chemical reaction in which a dye molecule absorbs ultraviolet photons, changes either chemically or conformationally, and the reaction product has an altered absorption characteristic of visible light. These familiar eyeglasses become dark in bright sunlight, and return to clear when indoors in a dimmer environment. These familiar devices have the drawback that the degree to which the absorption changes is controlled entirely by the intensity of ambient light, and not by the wearer.
Anti-glare sunglasses always perform their function. That is, they always differentially absorb light depending on its state of polarization, regardless of whether the effect is desired or not. The wearer of such eyewear frequently must remove the device when the ambient light level becomes dimmer, as when clouds cover the sun or when going indoors in order to see adequately in the dimmer environment. This may even become a safety hazard, as when entering a tunnel while driving on a sunny day.
Photochromic sunglasses only control the level of light transmission when the ambient light level is high, but they offer no protection against glare. Thus, although the amount of light entering the eye is reduced, this reduction alone may not result in clearer vision. Moreover, passive photochromic devices—those that rely solely on the absence or presence of ultraviolet light to change states—are very slow to change states. Typical photochromic sunglasses take ten to fifteen minutes to revert from a dimmed state to a bleached state. Notable prior art, U.S. Pat. No. 4,549,894, describes photochromic glass that regains a transmissivity 1.75 greater than it possesses in the fully darkened state 300 seconds after the activating illumination is removed. A variation on eyewear exhibiting this functionality exists, such as disclosed in U.S. Pat. No. 5,552,841, but it employs electro-optic means of controlling the light transmission in conjunction with electric-eye type devices. Such a device is complicated to manufacture, and still provides no glare protection.
Liquid crystal light shutters have also been executed as light transmission elements for eyewear. Some notable prior art is described in U.S. Pat. No. 4,279,474. In these devices, the electrically controllable birefringence of liquid crystals is exploited by sandwiching them between polarizers. In this execution, the light transmissivity of the eyewear is controlled via an external electrical signal. Often, this signal is slaved to a photo sensor to produce responsive eyewear. A familiar example of this are the “automatic” windows in welding helmets that rapidly darken when an arc is struck, protecting the wearer's vision, as described in U.S. Pat. No. 4,039,254.
U.S. Pat. No. 3,653,863 discloses an optical device capable of reversibly changing from a clear unpolarized state to a darkened polarized state upon exposure to actinic radiation. Such glasses are manufactured from a silicate glass body having elongated silver halide particulars incorporated therein and wherein the orientation of the halide particulars is accomplished by stretching the glass during manufacturing. Although this glass material is effective in its stated purpose, manufacturing of the glass in such a manner is somewhat cumbersome. Moreover, the performance of such a glass is considered to be unacceptable. As noted in the '863 disclosure, transmission in the undarkened state is approximately 77%, whereas in the present invention, transmission in the undarkened or bleached state can be greater than 85%, with the majority of the loss in transmissivity coming from reflective losses that are inherent to any optical device. Other disadvantages of this silver halide glass material are that the performance characteristics are poor and cannot be easily improved upon because when the concentration of silver halide is increased too much, the crystallites become too large and the glass becomes foggy. In the context of eyewear, glass is much heavier than plastic. As such, although the performance properties of the silver halide glass are desirable, the weight of eyewear using glass material is a significant detriment. While the percent polarization of the device described in U.S. Pat. No. 3,653,863 approaches 86%, that of the present reduction-to-practice is about 42% at the 620 nm wavelength. However, the greater percent polarization of the prior art comes at the cost of much greater absorption in the bleached state.
Therefore, there is a need in the art to provide both the capabilities listed in the abovementioned prior art by providing an optical device that differentially absorbs light according to its state of polarization, and performs this task automatically as a function of the intensity of ambient light. This effect is thus referred to as “photo-induced dichroism.” There is also a need for this device to be passive and, therefore, impervious to any failure other than being physically broken or having its constituents degrade chemically. If precise active control of the absorption rate is desired, the optical device may be electrically controlled.